This invention relates to golf balls and more particularly to an improved golf ball cover useful in making balls, particularly two-piece balls, having superior short iron and other playability characteristics.
Balata and blends of balata with elastomeric or plastic materials were for many years the materials of choice in the manufacture of covers for top grade golf balls These materials have good molding properties and accordingly could be readily compression molded about a spherical wound core to produce a high quality golf ball. An experienced player can apply spin to a balata covered wound ball such that it will fade or draw in flight or have the backspin necessary to stop aburptly on the green. These playability properties are most important in short iron play and can be exploited significantly only by relatively skilled players.
Balata and its synthetic substitutes, trans polybutadiene and trans polyisoprene, have today essentially been replaced by new materials. With the exception of a few lines of golf balls distributed through pro shops to professional golfers and those who would emulate them, newer synthetic polymers are the cover materials of choice.
Of the new synthetics, by far the most commonly used are a line of ionomers sold by E. I. Dupont de Nemours & Company under the trademark SURLYN. These materials comprise copolymers of olefins, typically ethylene, with an alpha, beta, ethylinically unsaturated carboxylic acid such as methacrylic acid. Metal ions such as sodium or zinc are used to neutralize some portion of the acidic groups in the copolymer resulting in a thermoplastic elastomer which has several advantages including a cost advantage over balata. The ionomers may be manufactured with a wide variety of properties by altering the identity of the comonomers, the fraction of the polymer comprising the carboxylic acid, the molecular weight of the polymer, the degree of neutralization of the polymer, and the identity of the metal ions used. Control of these parameters results in ionomer resins of different melt index, hardness, resilience and other mechanical properties which, in a golf ball cover, affect cut resistance, shear resistance, general durability, and resilience.
U.S. Pat. No. 3,819,768 to R. P. Molitor discloses that blends of sodium neutralized ionomer resins with zinc neutralized ionomer resins, as a class, have certain advantages which have not been achievable in any other way. Among these is the production of an unexpectedly high coefficient of restitution of golf balls having the blended ionomer cover. Such covers also resist cold cracking, have excellent aging properties, and are unexpectedly durable. The development of the SURLYN blended cover has been a major factor in the production of two-piece balls having covers which for all practical purposes cannot be cut in play, and which travel further when hit than any other USGA regulation ball as measured by controlled tests when hit by golfers or testing machines.
While the balata-covered, thread-wound balls are easily cut and very expensive, they nevertheless have excellent short iron playability. It is much more difficult to impart spin to an ionomer covered two-piece ball. Frequently, experienced players note that the ionomer covered two-piece balls have an unsatisfactory "feel".
The patent literature is replete with proposed cover formulations seeking to improve upon the balata and ionomer covers which have been commercially successful. Polyurethanes, thermoplastic rubbers, various block copolymers, polyesters, and polyamides, as well as various blends including such materials, have been proposed. Examples include: U.S. Pat. Nos. 3,359,231, 4,398,000, 4,234,184, 4,295,652, 4,248,432, 3,989,568, 3,310,102, 4,337,947, 4,123,061, and 3,490,246.
The manufacture of two-piece balls i.e., balls comprising a solid, molded, resilient core and a cover, has many significant advantages over the more expensive wound balls. There is accordingly a need for two-piece balls having short iron playability characteristics comparable to wound, balata-covered balls. Such covers must have an appropriate hardness to permit the accomplished golfer to impart proper spin. In addition, it should have a resilience when formulated to have the proper hardness value consistent with a high coefficient of restitution. Also, it must be readily manufactured in large volumes at low cost, have an appropriate specific gravity, and have a desireable white color.